Universal Serial Bus (USB) is an industry standard that defines the cables, connectors and communications protocols used in a bus for connection, communication, and power supply between computers and electronic devices. USB was designed to standardize the connection of computer peripherals (including keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters) to personal computers, both to communicate and to supply electric power. It has become commonplace on other devices, such as smartphones, PDAs and video game consoles. USB has effectively replaced a variety of earlier interfaces, such as serial and parallel ports, as well as separate power chargers for portable devices.
A new USB Power Delivery Specification is under development to enable delivery of higher power over new USB cables and connectors. This technology creates a universal power plug for laptops, tablets, etc. that may require more than five volts using cables and plugs compatible with existing USB solutions. The USB Power Delivery (USB-PD) Specification defines a communication link between ports connected via a USB-PD cable and USB-PD connectors. The communication is designed to be half-duplex and packet-based. The packets contain information that enables the two ports to communicate and negotiate the voltage and current the source port will provide to the sink port. The ports can also negotiate to switch roles (source to sink and vice versa). The underlying communication in the USB-PD specification can use binary frequency-shift keying (FSK) or biphase mark coding (BMC). This communication happens independently from the normal USB communications that appear on data wires such as D+ and D− per the USB standard. The USB-PD communication goes over a different wire (e.g., the Vbus or CC wire) rather than the USB data wires.
In USB power delivery (PD) there are four kinds of devices defined: provider-only, provider/consumer, consumer/provider, and consumer-only. Devices that are provider-only, provider/consumer, or consumer/provider may sometimes act in a source role—meaning they are providing DC voltage on the Vbus wire for the far-end device to consume or sink. This disclosure is relevant for provider-only, provider/consumer and consumer/provider USB-PD devices that are acting in a source role. The provision of power by a USB-PD source device to a consuming device over a USB cable consumes appreciable amounts of power in the USB-PD source device. Thus it would be beneficial to implement power-saving measures in such devices.